This application is related to U.S. Ser. No. 07/749,897, abandoned, which is hereby incorporated by reference in its entirety herein.
1. Field of the Invention
This invention relates generally to telecommunications systems. More particularly, it relates to digital multiport multidrop systems where a plurality of hosts are coupled via a single line to a digital telecommunications network and communicate with data terminals, where a first group of data terminals are coupled via a first single line to the digital telecommunications network and a second group of terminals are coupled via a second single line to the digital telecommunications network.
The invention is particularly applicable in the banking industry where it may be desirable for banks to have information from tellers, ATMs and security systems on a single line at various branches communicating with separate hosts for tellers, ATMs and security systems at a single different location.
2. State of the Art
Different multiport multidrop systems have been proposed. U.S. Pat. No. 4,858,230 to Duggan, for example, discloses an analog system where outbound messages from hosts are multiplexed and sent on a constant carrier over a single line to data terminals. Return messages from data terminals are similarly sent. However, since only one data terminal can use the line at any given time, responses from different terminals need to be delayed.
Recently, digital multiport multidrop systems have been proposed by Racal-Milgo and Paradyne. Digital systems do not use a carrier. Rather, as suggested by prior art FIG. 1, data from multiple hosts 12a, 12b, 12c are multiplexed by a multiplexer 14 and are output by a data service unit (DSU) 16 in digital format to the digital network 22. The digital network, which is maintained by the telephone company, includes a number of office channel units (OCUs) 24a, 24b, 24c, 24d, etc. and at least one multiple junction unit (MJU) 28. The MJU combines data from the OCUs (e.g., 24b, 24c, 24d) based on information supplied to the MJU through the OCU. The OCU tells the MJU whether the OCU is in data send mode or in an idle mode (based on whether the OCU 24 is receiving data from any of the drops 42a-42i of ports 43a-43c via multiplexer/demultiplexers 44a-44c and DSUs 36a-36c). There are essentially two ways in which the OCUs can tell the MJU which branches are idle. These are referred to as "polling disciplines", as defined in AT&T Publication 62310, page 27 (1987).
The two known polling disciplines are often referred to as "data mode idle" and "control mode idle". In the case of data mode idle, OCUs signal an idle state by transmitting continuous digital ones. Data bits received by an MJU from the OCUs are combined in a logical AND so that all of the OCUs supplying continuous logical ones are effectively idle and the OCU supplying a varying bit stream of ones and zeros is passed through by the MJU. Data mode idle has a disadvantage, though. If there is a bit error from any one of the idle OCUs, it is combined with the data stream from the active OCU and thus, the opportunity for corrupted data is enhanced. Control mode idle avoids the possibility of data corruption by consigning the MJU to ignore idle channels. In control mode idle, an OCU signals the MJU that it is idle via network signalling known in the art. So long as the idle sequence is received, data from that OCU is ignored (i.e., not ANDed).
In any polled application, it is the responsibility of the control (master) station to guarantee that only one tributary station (remote terminal) responds at any one time. Therefore, it is true to say that Drop-1 channel-1 will always be inactive while Drop-2 channel-1 is responding or vice versa. But with a number of independent applications running simultaneously, one at each channel, collision between channels (e.g. Drop-1 channel-1 and Drop-2 channel-2) is a real possibility.